Method of providing malleable metal coatings on particles of lubricants



Se t. 23, 1969 R. 0. KREMITH 3,463,699

METHOD OF PROVIDING MALLEABLE METAL COATINGS 0N PARTICLES OF LUBRICANTS FiledOct. 14. 1966 -L UBQ/CANT AND M57394 CVfl/V/DE 0E N/ TRADE POM/DEBS D/Q Y /0 V M/ L L ,PEACV'ED 7725/1750 5 4,95 WATER 04/403 44s WASTE 70 SEWER Z? couzc: TION 7725A TMENT SAL/D65 2773- TO 27 Ln0o- POWDER INVENTOR.

20; AND D. KEEM/ 7w Afro/avers United States Patent US. Cl. 117-100 7 Claims ABSTRACT OF THE DISCLOSURE A method of forming composite particles the cores of which are lubricants, and the coatings of which are malleable metals. The core particles are first thoroughly mixed with coating particles selected from a class consisting of the cyanides and the nitrides of the malleable metals. Thereafter, the mixed core and coating particles are heated in a fluidized-bed reactor until the reduction temperature of the coating particles is achieved. Hydrogen gas is then passed in contact with the mixed particles in order to reduce the coating particles and form coatings of the malleable metals on the core particles.

This application is a continuation-in-part of my copending application Ser. No. 579,579, filed Sept. 15, 1966, for Method of Forming a Solid-Film Lubricant Coating Containing Graphite, now abandoned.

This invention relates to a method of manufacturing nonmetallic lubricant particles which are individually coated with malleable or ductile metals, and to the coated particles thus formed.

As indicated in the above-cited patent application Ser. No. 579,579, extremely satisfactory lubricant coatings, having the capability of resisting relatively high temperatures, may be formed by the plasma spraying of lubricant particles to which malleable or ductile metal coatings have previously been applied. In accordance with the present invention, the metal-coated lubricant particles are manufactured by a fluidized-bed process making use of the cyanides and the nitrides of the desired malleable metals.

A prior-art reference is an article entitled Tungsten and Molybdenum Coated Nonmetallic Powders, published in the August 1965, issue of the Journal of Metals, Written by A. Landsberg, T. T. Campbell and F. E. Block. The process there described contemplates the use of a fluidized bed and the reduction of metal chlorides or oxides to form tungsten and molybdenum coatings on refractory particles. Such process requires processing temperatures which are excessively high for use in coating lubricants. Thus, for example, the 900-1000 degreeC. temperature required by the described process would effect decomposition of sulfide-based lubricant compounds. In addition, and with particular relation to the oxide process described in the article, undesirable contamination would be introduced even if coating of lubricant particles could be achieved.

In view of the above and other factors, it is a primary object of the present invention to provide a method of achieving fluidized-bed coating of particles of non-metallic lubricants with malleable metals at temperatures sufiiciently low to prevent substantial decomposition or other adverse effects, and without introducing any significant degree of contamination.

A further object of the invention is to provide improved particles adapted to form lubricant coatings upon a substrate, such particles comprising cores of graphite which have been coated with silver, copper or mixtures thereof.

These and other objects will become apparent from the following detailed description considered in connection with the accompanying single-figure drawing which comprises a flow diagram of the present method.

The cores, or base particles, which are individually coated by the present method comprise suitable nonmetallic lubricants, namely graphite and molybdenum disulfide. Such particles or cores are normally irregular in shape, and typically have diameters in the range of about 10 to about 53 microns.

As is well known, both graphite and molybdenum disulfide are excellent dry lubricants. However, both tend to decompose (or chemically react) when employed for substantial periods of time at elevated temperatures. When coated with malleable metals in accordance with the present method, both graphite and molybdenum disulfide not only achieve excellent resistance to elevated temperatures but also are readily sprayed onto a substrate by means of an electrical plasma-jet torch. The spray methods are described in detail in the cited patent applications.

The malleable or ductile metals which are coated onto the lubricant particles, in accordance with the present method, preferably comprise copper or silver. Other malleable metals which may be employed are nickel, iron, zinc, manganese, cadmium and lead.

As previously indicated, the present invention contemplates providing coatings of the above-indicated (or other) malleable metals onto lubricant particles by a fluidizedbed process making use of the cyanides or nitrides of the metals.

Referring to the single-figure drawing, the fluidizedbed process may be performed by mixing, with the lubricant particles (the core particles), particles of the metal cyanide or nitride (the coating particles) to be employed in the particular reaction. Such mechanical mixing may be achieved in the dry mill (for example, a ball mill) indicated at 10. Milling is preferably continued for a substantial time period, so that thorough mixing will occur. Thus, for example, the ball milling may be continued for a substatnial number of hours.

It is to be understood that, although only one type of lubricant particle will normally be mixed with only one metal cyanide or metal nitride, more than one type of lubricant may be employed in certain instances. Different metal cyanides or metal nitrides may also be employed, it being understood, however, that the cyanides are not normally intermixed with the nitrides.

The resulting mixed core particles and coating particles are screened, in the screen indicated at 11, to the desired particle size (in the range of 10 to 53 microns, as previously indicated). A preferred particle size is on the order of 44 microns.

The screened particles are then introduced, as through conduit means 12 and a valve indicated at 13, into a fluidized-bed reactor 14 which is constructed with suitable heating means, not shown. The heating means may comprise, for example, suitable resistance-heating elements, induction-heating elements, etc. The lower end of the bed 14 communicates through a conduit means 16 and valve means 17 with the discharge region at which the coated powder is withdrawn.

A suitable source 18 of pressurized argon gas (or other suitable inert gas) communicates through a valve 19 and cross connector 20 with the conduit means 16 (between valve 17 and bed 14). Furthermore, a source 21 of pressurized hydrogen gas communicates through a valve 22 with cross connector 20 and thus with the conduit means 16.

The fluidized bed 14 communicates, at its upper end, with an exhaust conduit means 23 leading to a suitable gas-collecting means 24 which, in turn, communicates with a waste treatment means 25. The waste treatment means 25 communicates through a conduit 26 with a sewer adapted to receive waste liquids, and through a second conduit 27 with a lagoon or other region adapted to receive sludge. The means 24 and 25 are adapted to receive both the argon from source 18 and the products of the reaction between the hydrogen (from source 21) and the metal cyanide or nitride in the bed 14. Means are preferably provided to separate the argon for re-use, and to treat the exhaust products (resulting from the reaction between the hydrogen and the cyanide or nitride) to thus render the same relatively harmless.

After the fluidized-bed reactor 14 has been charged with a suitable quantity (preferably about one-half the capacity thereof) of mixed powders from mill via screen 11, valve 13 is closed, valves 17 and 22 are closed, and valve 19 is opened. Argon from source 18 is thus passed through valve 19 and conduit means 16 to the bed 14 boiling upwardly through the mixed powders to the exhaust conduit 23. During this operation, the heating means in the bed 14 is employed to bring the mixed powders up to a suitable reduction temperature of the cyanide or nitride compound contained therein. The bed 14 should be suitably agitated as by a mechanical vibrator, not shown, in order to prevent bridging of the particles, chimneying of the gas flow, etc.

It is a feature of the present method that the chemical reduction temperatures of the cyanide or nitride salts employed are low, namely in the range of about 200 degrees C. to about 900 degrees C. Thus, the reduction temperature may be reached without causing substantial decomposition of the lubricant (core) powder. However, it has been found that the reduction temperatures should be close to (but below) the decomposition temperatures of the cyanide or nitride salts, so that reduction will occur rapidly.

After the desired reduction temperature is reached, valve 22 is opened to permit hydrogen gas to flow from source 21 through conduit 16 into the bed and thence into the exhaust conduit 23. During this operation, valve 19 is preferably progressively closed, so that flow of argon is diminished as hydrogen gas is added. The hydrogen reacts with the metal cyanide powder to reduce the same, the pure malleable metal then being coated onto the individual particles of lubricant. Heating is then discontinued, but flow of hydrogen gas is continued in order to effect cooling of the particles.

The products of the reaction are normally hydrocyanic acid or nitric acid, that is to say the vapors of such acids. The vapors are treated in the gas collection and waste treatment means 24 and 25 indicated above.

After completion of the coating (and subsequent cooling) process, valves 19 and 22 are closed and valve 17 is opened to effect discharge of the coated particles from the bed 14. The resulting lubricant particles, coated with the desired malleable metal, are then employed for various purposes including plasma deposition (by means of an electrical plasma-jet spray torch) onto a suitable substrate to thus form a lubricating coating. Because of the malleable metal coatings on the individual lubricant particles, the ability of such lubricant particles to lubricate successfully (even in an oxidizing atmosphere) at high temperatures is greatly extended. For example, coatings formed of the present composite particles will lubricate successfully up to about 1800 degrees F. This compares to only about 900 degrees F. for coatings formed of uncoated particles.

It is to be understood that, as indicated above, any combination of the various malleable metals may be produced., Control of the ratio of coating weight to core weight is achieved by controlling the Weight ratios of the blend in dry mill 10.

It is an important feature of the present method that the malleable metal coatings are substantiall entirely free of contaminants, all reaction products discharging through the exhaust conduit 23. Stated otherwise, the reaction products are light, and readily fiow or boil away. This substantial purity of the deposited metals is important for many reasons, one of which is that certain ofthe reaction products are dangerous (for example, hydrocyanic acid).

The cyanides of the malleable metals are suitable for coating both graphite and molybdenum disulfide. Thus, for example, copper may be formed from cuprous cyanide (CuCN), silver from silver cyanide (AgCN), nickel from nickel cyanide (Ni(CN) -4H O), iron from ferrocyanide (Fe (Fe(CN) zinc from zinc cyanide (Zn(CN) and magnesium from magnesium cyanide (MgCN).

The cyanide reactions which take place are as follows:

The nitrides may be employed in coating molybdenum disulfide but at least some nitrides are not presently regarded as satisfactory for providing malleable metal coatings on graphite particles. Thus, the nitrates of silver, copper, and gold, when used to coat graphite, produce explosive acetylide compounds. The metal coating may be formed by deriving silver from silver nitrate (AgNO cadmium from cadminum nitrate (Cd(NO lead from lead nitrate Pb(NO magnesium from magnesium nitrate (Mg(NO -2H O) and nickel from nickel nitrate (Ni(NO -6H O).

A typical reaction, which takes place from the hydrogen reduction of the metal nitrite, is as follows:

A 2 AgNO: H: 2 Ag ZHNO;

(with a liquid (HNO phase which is boiled off during the process).

Itis within the scope of the invention to separately heat the core powders, separately heat the metal nitride or metal cyanide powders, and thereafter mechanically mix such powders while the same remain in heated condition.

.The mixed powders should be dry. Therefore, the particles may be oven dried (after screening) for a period of one hour, for example.

. The present invention also comprises certain composite powders as such, as distinguished from the method of making the same. Such composite powders constitute cores of graphite which have been coated with copper, silver or mixtures thereof.

Specific example No. 1

As a first specific example, cuprous cyanide (a white crystalline powder which melts with decomposition at 473 degrees C.) was used to supply copper coating metal onto graphite cores. The coating-to-lubricant weight ratio, and the amount of powder to be prepared, werefirst selected. For this purpose, the following formula was employed:

Percent Cu Percent; graphite Molecular wt. CuCN In the present example, 400 grams of 35 percent-by Weight copper, coated onto 65 percent-by-weight graphite, was desired. Employing the above formula, the weight of graphite was 260 grams, and the weight of cuprous cyanide was 197 grams.

The calculated quantities of graphite and cuprous cyanide powders, in the size range of --200+325 mesh, were blended and placed in a one-quart ball mill, with balls. The mill was operated for from six to eight hours at 100 revolutions per minute. Thereafter, the resulting mechanically-mixed powders were screened through a 200 mesh screen, and powders which did not pass through the screen were re-milled. This process was continued until all material passed through the screen. The material was then oven dried, at a temperature of 100 degrees C., for one hour.

The mechanically-mixed and screened powders were then placed in a fluidized bed having a capacity of approximately one liter. Argon gas was passed through the bed, as described above, the flow rate being from two to eight cubic feet per hour (for example, approximately five cubic feet per hour). The fluidized bed was heated to an inside temperature of from 460 to 480 degrees 0, following which hydrogen gas was introduced at a rate of from about five to about fifteen cubic feet per hour (for example, ten cubic feet per hour), the flow of hydrogen gas being continued for 15 minutes. As hydrogen gas was added, the rate of flow of argon was progressively diminished. A suitable heater control was employed to maintain a constant temperature, in the fluidized bed, during the hydrogen flow period. Thereafter, heating was discontinued and the bed -was cooled to ambient, employing hydrogen gas as the fluidizing medium. The resulting copper-coated graphite was then removed from the bed.

Specific example No. 2

In this specific example, graphite was coated with silver. To accomplish this, silver cyanide (a white crystalline material which melts with decomposition at 320 degrees C.) was employed. The same process was employed as specified above, the only difierence being that the bed tempearture was caused to be from 300 degrees C. to 320 degrees C. Employing the formula stated in the preceding specific example, and since it was desired to produce 400 grams of composite particles, the composite particles being 60 percent-by-weight silver and 40 percent-by-weight graphite, the following particle weights were determined: 160 grams of graphite, and 298 grams of silver cyanide.

Specific example No. 3

To prepare molybdenum disulfide powder having a silver coating, silver nitrate (a colorless salt which melts at 212 degrees C. and decomposes at 440 C.) was employed. Such salt, when heated to approximately its melting point, is readily reduced with hydrogen to form elemental silver, water and nitric acid. A typical reaction which takes place is as follows:

2AgNO3 2Ag 2HNOs The process followed was identical to that specified in specific example No. 1, above, except that the bed temperature was 400 to 450 degrees C. To prepare 400 grams of silver coated molybdenum disulfide, comprising 30 percent-by-weight silver coated onto 70 percent-byweight molybdenum disulfide, 280 grams of molybdenum disulfide, and 189 grams of silver nitrate, were required.

The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.

I claim:

1. A method of forming composite particles having non-metallic lubricant cores and malleable metal coatings, which method comprises:

providing solid core particles formed of at least one non-metallic lubricant substance,

dry mixing, with said core particles, coating particles selected from a class consisting of the cyanides and the nitrides of malleable metals, heating said mixed core particles and coating particles in a fluidized-bed reactor until the reduction temperature of said coating particles is achieved, and

passing hydrogen gas in contact with said mixed particles to thereby reduce said coating particles and form coatings of the malleable metals on said core particles.

2. The invention as claimed in claim 1, in which said core particles are selected from a class consisting of graphite and molybdenum disulfide.

3. The invention as claimed in claim 1, in which said coating particles are cyanides selected from a class consisting of the cyanides of copper, silver, nickel, iron, zinc and magnesium.

4. The invention as claimed in claim 3, in which said core particles are selected from a class consisting of graphite and molybdenum disulfide.

5. The invention as claimed in claim 1, in which said coating particles are nitrides selected from a class consisting of the nitrides of nickel, silver, cadmium, lead and magnesium.

6. The invention as claimed in claim 5, in which said core particles are molybdenum disulfide.

7. The invention as claimed in claim 1, in which said heating step effects heating of said mixture to a reducing temperature in the range of 200 to 900 degrees C.

References Cited UNITED STATES PATENTS 2,773,844 12/1956 Carlson et al. 117-100 X 2,853,398 9/1958 Mackiw et al. 117100 X 3,144,472 8/1964 Werner et al. 117-lO7.2 X 3,178,308 4/1965 OXley et al. 117-107.2 X 3,198,735 8/1965 Lamson 252--30 X 3,223,523 12/1965 Adler 117-100 X 3,288,623 11/1968 Colton 117-10O X 3,343,953 9/1957 Schladitz 1l7-100 X FOREIGN PATENTS 821,062 9/1959 Great Britain.

OTHER REFERENCES Landsberg et al., Journal of Metals Tungsten and Molybdenum Coated Nonmetallic Powders, pp. 850'- 855, August 1965.

WILLIAM D. MARTIN, Primary Examiner MATHEW R. P. PERRONE, JR., Assistant Examiner US. Cl. X.R. 

